Monometallic and Bimetallic Catalysts Supported on Praseodymium-Doped Ceria for the Water–Gas Shift Reaction

The water–gas shift (WGS) performance was investigated over 5%Ni/CeO2, 5%Ni/Ce0.95Pr0.05O1.975, and 1%Re4%Ni/Ce0.95Pr0.05O1.975 catalysts to decrease the CO amount and generate extra H2. CeO2 and Pr-doped CeO2 mixed oxides were synthesized using a combustion method. After that, Ni and Re were loaded onto the ceria support via an impregnation method. The structural and redox characteristics of monometallic Ni and bimetallic NiRe materials, which affect their water–gas shift performance, were investigated. The results show that the Pr addition into Ni/ceria increases the specific surface area, decreases the ceria crystallite size, and improves the dispersion of Ni on the CeO2 surface. Furthermore, Re addition results in the enhancement of the WGS performance of the Ni/Ce0.95Pr0.05O1.975 catalyst. Among the studied catalysts, the ReNi/Ce0.95Pr0.05O1.975 catalyst showed the highest catalytic activity, reaching 96% of CO conversion at 330°. It was established that the occurrence of more oxygen vacancies accelerates the redox process at the ceria surface. In addition, an increase in the Ni dispersion, Ni surface area, and surface acidity has a positive effect on hydrogen generation during the water–gas shift reaction due to favored CO adsorption.


Introduction
Water-gas shift (WGS) reaction is an industrial technology that involves the reaction of water vapor and carbon monoxide to generate H 2 and CO 2 .The water-gas shift reaction takes place according to the following equation: The development of H 2 and fuel cell technologies provides numerous advantages such as high-efficiency power, environmentally friendliness, and sustainability.WGS reactions have received widespread attention to increase the H 2 concentration in the syngas.Hydrocarbon reforming can produce syngas that consists of CO, H 2 , H 2 O, and CO 2 .However, a trace concentration of CO poisons the catalysts utilized in fuel cells [1].Precious metalbased proton exchange membrane fuel cell anodes require a carbon monoxide amount in the inlet gas below 10-20 ppm; otherwise, the anode is poisoned [2].A purification process is required to decrease the CO concentration lower than the cell tolerance level after hydrogen is produced via the reforming process of carbon-containing molecules (such as hydrocarbons or alcohols).The advantage of using the WGS reaction is to reduce CO content while generating more hydrogen as fuel for the H 2 fuel cell.The appearance of a suitable catalyst in the WGS reaction can reduce the CO content to 10 ppm.
The support plays a critical role in oxidation reactions such as water-gas shift reactions or CO oxidation.The utilization of redox-active oxides such as CeO 2 as a support material leads to superior catalytic efficiency compared to the use of other oxide support such as alumina or silica [3][4][5][6].Due to its redox characteristics, CeO 2 can promote vacancy generation and water dissociation, which plays an important role in the catalytic performance of water-gas shift reactions.However, pure CeO 2 has some disadvantages, like deactivation of the Ce 4+ /Ce 3+ redox couple and thermal sintering, resulting in a reduction in its oxygen storage capacity (OSC) and catalytic performance [7].Thus, many efforts have been dedicated to improving the reducibility and catalytic performance of CeO 2 by doping it with other cations to generate more oxygen vacancies and develop its resistance to thermal sintering [8,9].In this part, doping CeO 2 with aliovalent (such as Eu 3+ , La 3+ , Gd 3+ , and Sm 3+ ) cations [10][11][12] or isovalent (such as Zr 4+ , Hf 4+ , and Ti 4+ ) cations [13,14] has been well described in the literature.The incorporation of an aliovalent cation into the CeO 2 lattice produces oxygen vacancies by charge compensation on the final oxide materials [15].Conversely, the CeO 2 doping with isovalent cations also results in the enhancement of the redox properties of CeO 2 .Therefore, the partial substitution of Ce 4+ by Zr 4+ causes a deformation in the CeO 2 lattice because of the lower ionic radius of isovalent cations such as Zr 4+ , deriving an enhancement in the OSC of CeO 2 .Moreover, these two effects can also be combined in a single study when doping CeO 2 with variable oxidation states of lanthanide elements.In such cases, Tb [16] and Pr [17,18] are the most studied ones.The Pr addition into the CeO 2 lattice enhances both oxygen desorption and oxygen vacancy generation compared to pure CeO 2 .This result is due to the lower binding energy of oxygen anions in Ce-Pr mixed oxides and the higher reduction potential of Pr 4+ /Pr 3+ compared to Ce 4+ /Ce 3+ .Therefore, the Zr dopant induces a limited amount of oxygen vacancies into the CeO 2 lattice [19,20], whereas the Pr dopant leads to a greater concentration of oxygen vacancies [21].These behaviors result in redox properties in the Ce-Pr mixed oxides being superior to those of other mixed oxides.In previous works, Ce-Pr-O mixed oxide supports were studied for WGS reaction.It was found that the optimum promoting effect of Pr appears at a low loading of 5 %wt.[22,23].In addition, the catalytic activity of such oxides can be obviously improved by adding a small amount of transition metals (cobalt, chromium, copper, and nickel) [24][25][26].Nickel combined with CeO 2 -based oxides are costeffective alternatives to expensive noble metal catalysts and are often more reactive than noble metals [27].However, CO and CO 2 methanation are very common side reactions for these catalysts [28,29].This behavior is usually tempered by the incorporation of a second active metal like platinum [30,31].Rhenium might be a good choice for replacing Pt catalyst because of its good electrochemical properties, its cheaper price compared to platinum, and its sustainable source.Re is widely used as a second metal to form bimetallic catalysts.In recent years, the use of bimetallic catalysts has attracted much attention due to their excellent efficiency and capability [32][33][34].The effect of rhenium on the WGS performance of Pt/TiO 2 and Pt/ZrO 2 catalysts has been investigated, and the results exhibited that rhenium addition induces an increase in the WGS activity of Pt catalysts.Rhenium acts as an anchor for the platinum particles to enhance the Pt dispersion.In addition, the redox process between Re 4+ and Re 7+ over the WGS reaction would promote CO oxidation on the Pt catalyst [35,36].Our initial studying found that rhenium also enhanced the WGS activity when it was doped on Ni/CeO 2 -based oxides [11,37,38].Apart from enhancing the performance of the WGS reaction, Pt-Re/carbon has also been reported to be active in several reactions, such as the reforming process [23] and glycerol to syngas conversion [39].Furthermore, in studying the catalytic activity of ReCo/Al 2 O 3 for the Fischer-Tropsch reaction found that Re addition increased the reaction rate of the Co catalyst.Re has been shown to be a good promoter by facilitating the reduction rate of cobalt species and producing more available active Co metal sites to participate in the reaction [40].
In this work, the performance of Ni/CeO 2 and Ni/Ce 0.95 Pr 0.05 O 1.975 catalysts for the water-gas shift reaction was studied.Additionally, the role of rhenium addition on the water-gas shift performance of Ni/Ce 0.95 Pr 0.05 O 1.975 catalyst was also observed.Therefore, the utilization of Pr as a dopant and Re as a metal additive in this work to maximize the WGS performance.The physicochemical properties of monometallic and bimetallic catalysts were examined to clarify the key factors in increasing the catalytic activity using the following techniques: X-ray diffraction, BET surface area, NH 3 temperature programmed desorption, H 2 temperature programmed reduction, Raman spectroscopy, and chemisorption techniques.when compared with Ni/CeO 2 is due to Ce 4+ ions (0.097 nm) being replaced by larger Pr 3+ ions (0.112 nm).Therefore, an oxygen vacancy is expected to be formed because Pr 3+ incorporation in the CeO 2 lattice produces unbalanced charges and strain.On the other hand, the diffraction peaks of Ni/CeO 2 appeared at higher diffraction angles compared with the diffraction peaks of pure CeO 2 because of the lattice contraction after calcination at high temperatures.Nickel could not be incorporated into CeO 2 lattice due to the nature of impregnation synthesis but the reduction in the cell dimension because of the decomposition of surface hydroxyls during calcination at 650 • C.

Results and Discussion
Molecules 2023, 28, x FOR PEER REVIEW 3 of 16 desorption, H2 temperature programmed reduction, Raman spectroscopy, and chemisorption techniques.

Catalysts Characterization
X-ray diffraction patterns of CeO2, 5%Ni/CeO2, 5%Ni/Ce0.95Pr0.05O1.975,and 1%Re4%Ni/Ce0.95Pr0.05O1.975were illustrated in Figure 1.It suggests that the diffraction peaks of all catalysts correspond well to CeO2 phases with a cubic structure (Joint Committee on Powder Diffraction File No. 43−1002).In addition, the weak peaks at 2θ about 37.1°, 43.2° and 63.2° attributed to the NiO phases of Ni−based catalysts, suggesting that there was a small proportion of nickel oxide.The CeO2 crystallite size of supported Ni catalysts was determined using the Debye-Scherrer equation (Table 1).The calcination of Ni/CeO2 catalyst at a high temperature (650 °C) after impregnation of Ni onto ceria support leads to the aggregation of ceria crystallites; thereby, the surface area decreases with a growth in CeO2 crystallite size.However, Pr addition into 5%Ni/CeO2 results in a decrease in the ceria crystallite size together with an increase in a specific surface area.The diffraction peaks of Ni/Ce0.95Pr0.05O1.975and ReNi/Ce0.95Pr0.05O1.975appeared at lower diffraction angles compared with the diffraction peaks of Ni/CeO2, indicating that Pr incorporation in the CeO2 lattice enlarged unit cell.The enhancement of the unit cell for Ni/Ce0.95Pr0.05O1.975when compared with Ni/CeO2 is due to Ce 4+ ions (0.097 nm) being replaced by larger Pr 3+ ions (0.112 nm).Therefore, an oxygen vacancy is expected to be formed because Pr 3+ incorporation in the CeO2 lattice produces unbalanced charges and strain.On the other hand, the diffraction peaks of Ni/CeO2 appeared at higher diffraction angles compared with the diffraction peaks of pure CeO2 because of the lattice contraction after calcination at high temperatures.Nickel could not be incorporated into CeO2 lattice due to the nature of impregnation synthesis but the reduction in the cell dimension because of the decomposition of surface hydroxyls during calcination at 650 °C.H2 chemisorption analysis was used to determine the Ni dispersion of supported Ni catalysts.It was found that an addition of Pr to Ni/CeO2 increases Ni dispersion on the catalyst surface.Moreover, rhenium impregnation onto Ni/Ce0.95Pr0.05O1.975tremendously enhanced the dispersion and surface area of metallic nickel.This result may be due to the movement of electrons between Re, Ni, and CeO2, which results in the formation of strong interaction between Ni metal and support; thereby, the metal dispersion and metal surface coverage enhances, whereas particle size reduces.1%Re4%Ni/Ce0.95Pr0.05O1.975exhibited the highest Ni surface area and dispersion among all the catalysts.Usually, a greater metal surface area provides more surface active sites exposed to reactants [36,37].enhanced the dispersion and surface area of metallic nickel.This result may be due to the movement of electrons between Re, Ni, and CeO 2 , which results in the formation of strong interaction between Ni metal and support; thereby, the metal dispersion and metal surface coverage enhances, whereas particle size reduces.1%Re4%Ni/Ce 0.95 Pr 0.05 O 1.975 exhibited the highest Ni surface area and dispersion among all the catalysts.Usually, a greater metal surface area provides more surface active sites exposed to reactants [36,37].The chemical analysis by SEM micrographs with the corresponding elemental mapping was conducted to investigate the elemental distribution and the homogeneity of the supported Ni catalyst.As presented in Figure 2, Ce, Pr, and Ni elements were uniformly distributed on 5%Ni/Ce 0.95 Pr 0.05 O 1.975 catalysts.The highly dispersed Ni suggests a strong metal-support interaction, and the enhancement in Ni dispersion would provide more active sites that are exposed to reactants, which is beneficial to the increase in WGS performance.
Molecules 2023, 28, x FOR PEER REVIEW 4 of 16 The chemical analysis by SEM micrographs with the corresponding elemental mapping was conducted to investigate the elemental distribution and the homogeneity of the supported Ni catalyst.As presented in Figure 2, Ce, Pr, and Ni elements were uniformly distributed on 5%Ni/Ce0.95Pr0.05O1.975catalysts.The highly dispersed Ni suggests a strong metal-support interaction, and the enhancement in Ni dispersion would provide more active sites that are exposed to reactants, which is beneficial to the increase in WGS performance.Raman spectroscopy was performed to quantify oxygen vacancies in the catalyst.CeO2 catalyst initiates the water-gas shift process via a redox mechanism at high temperatures.CO adsorbs on the catalyst surface and subsequently oxidizes it with CeO2 lattice oxygen to generate carbon dioxide and oxygen vacancy.H2O oxidizes reduced CeO2 again to produce H2.A mechanism for increasing the catalytic performance of CeO2 is the incorporation of dopant ions, with Pr 3+ as a promising candidate dopant [41].It is widely regarded that when doping CeO2 with a trivalent cation, two Ce 4+ ions in the CeO2 lattice are substituted by the dopants, and then an O ion is eliminated to conserve the charge [15].Therefore, oxygen vacancy is directly related to catalytic activity in water-gas shift reaction.Raman spectroscopy was performed to quantify oxygen vacancies in the catalyst.CeO 2 catalyst initiates the water-gas shift process via a redox mechanism at high temperatures.CO adsorbs on the catalyst surface and subsequently oxidizes it with CeO 2 lattice oxygen to generate carbon dioxide and oxygen vacancy.H 2 O oxidizes reduced CeO 2 again to produce H 2 .A mechanism for increasing the catalytic performance of CeO 2 is the incorporation of dopant ions, with Pr 3+ as a promising candidate dopant [41].It is widely regarded that when doping CeO 2 with a trivalent cation, two Ce 4+ ions in the CeO 2 lattice are substituted by the dopants, and then an O ion is eliminated to conserve the charge [15].Therefore, oxygen vacancy is directly related to catalytic activity in water-gas shift reaction.
As shown in Figure 3, a Raman peak near 460 cm −1 was attributed to a triple degeneracy active mode (F 2g peak), which represents the symmetrical stretching vibration generated by eight O atoms bound to one Ce atom.Secondary peaks at around 240 and 320 cm −1 characteristics of CeO 2 nanostructures are also found in 1%Re4%Ni/Ce 0.95 Pr 0.05 O 1.975 .In addition, another broad peak near 570 cm −1 (denoted by D peak) was associated with oxygen vacancies in CeO 2 [42,43].The oxygen vacancies concentration can be represented by the ratio of I D /I F2g [44].The intensity of the D peak in 1%Re4%Ni/Ce 0.95 Pr 0.05 O 1.975 catalyst is stronger than that of other catalysts, indicating that higher oxygen vacancy concentration can be obtained by the addition of Re onto Ni/Ce 0.95 Pr 0.05 O 1.975 catalyst.Moreover, the presence of Ce 3+ in the CeO 2 -based catalyst can be demonstrated by a red shift of the F 2g peak, which is due to the lattice expansion when Ce 4+ ions (ionic radius 0.097 nm) are replaced by Ce 3+ ions (ionic radius 0.114 nm) for oxygen vacancy formation [43,45]  In this case, electron density transfers between Re, Ni, Ce, and Pr may occur.As the result of electron density transfer, a concurrent reduction of metal oxide species was found, and reduction of metal oxide is easier.A stronger interaction between nickel and CeO 2 is expected to tune the nickel dispersion.The reduction peak appearing at a higher temperature normally means that it is more difficult to reduce with stronger metal-support interactions [48].For the 1%Re4%Ni/Ce 0.95 Pr 0.05 O 1.975 catalyst, stronger metal-support interactions are presented, which proved that the reduction of the surface shell of CeO 2 occurred at a higher temperature.The stronger interaction between metal and support is beneficial to maintain the metal dispersion and hinder its aggregation.The surface acidity of the prepared catalysts was studied using temperature-programmed desorption of ammonia (Figure 5), and the total acidity was estimated from the area under the NH 3 desorption peak.NH 3 -TPD analysis was carried out in order to clarify the effect of the acidity of ReNi/Ce 0.95 Pr 0.05 O 1.975 and Ni/Ce 0.95 Pr 0.05 O 1.975 on the catalytic performance in the water-gas shift reaction.The peaks were assigned to weak, medium, or strong acid sites when falling in the 100-200 • C, 200-450 • C, or 450-700 • C temperature range, respectively.Increased surface acidity enabled a higher content of CO adsorption on the catalyst surface since a CO reactant in the WGS process is a weak base, explaining the observed increase in catalyst activity.Furthermore, the acidic character of the Ni catalyst surface proved to be beneficial for CO 2 desorption, leaving behind free active sites for carbon monoxide and H 2 O adsorption in subsequent reaction cycles [49].The result from NH 3 -TPD analysis indicates that the addition of Re onto Ni/Ce 0.95 Pr 0.05 O 1.975 increases the concentration of weak-strength acid sites (peak area increases at <200 • C).In addition, the total concentration of surface acid sites can be estimated by integrating the NH 3 -TPD curves, and it was found to be 41 and 28 mols/g for 1%Re4%Ni/Ce 0.95 Pr 0.05 O 1.975 and 5%Ni/Ce 0.95 Pr 0.05 O 1.975 , respectively.The obtained results could imply a higher tendency for carbon monoxide adsorption and subsequently easier CO 2 desorption on the bimetallic ReNi supported by Pr-doped CeO 2 surface, thereby the overall water-gas shift reaction rate over ReNi/Ce 0.95 Pr 0.05 O 1.975 may be enhanced.
area under the NH3 desorption peak.NH3−TPD analysis was carried out in order to clarify the effect of the acidity of ReNi/Ce0.95Pr0.05O1.975and Ni/Ce0.95Pr0.05O1.975 on the catalytic performance in the water-gas shift reaction.The peaks were assigned to weak, medium, or strong acid sites when falling in the 100-200 °C, 200-450 °C, or 450-700 °C temperature range, respectively.Increased surface acidity enabled a higher content of CO adsorption on the catalyst surface since a CO reactant in the WGS process is a weak base, explaining the observed increase in catalyst activity.Furthermore, the acidic character of the Ni catalyst surface proved to be beneficial for CO2 desorption, leaving behind free active sites for carbon monoxide and H2O adsorption in subsequent reaction cycles [49].The result from NH3−TPD analysis indicates that the addition of Re onto Ni/Ce0.95Pr0.05O1.975increases the concentration of weak-strength acid sites (peak area increases at < 200 °C).In addition, the total concentration of surface acid sites can be estimated by integrating the NH3−TPD curves, and it was found to be 41 and 28 mols/g for 1%Re4%Ni/Ce0.95Pr0.05O1.975and 5%Ni/Ce0.95Pr0.05O1.975,respectively.The obtained results could imply a higher tendency for carbon monoxide adsorption and subsequently easier CO2 desorption on the bimetallic ReNi supported by Pr−doped CeO2 surface, thereby the overall water-gas shift reaction rate over ReNi/Ce0.95Pr0.05O1.975may be enhanced.XPS characterization was used to investigate the surface chemical states of the catalysts.Figure 6a shows the O 1s XPS spectra of 5%Ni/Ce0.95Pr0.05O1.975and 1%Re4%Ni/Ce0.95Pr0.05O1.975catalysts.Three different types of oxygen species were detected in all samples.The detected peaks near 529 eV (OL), 532 eV (OA), and 533 eV (OH) are attributed to lattice oxygen in metal oxide, chemically adsorbed oxygen on the surface, and a surface hydroxyl oxygen species, respectively.The ratios of OA/OL, which are calculated from the area of each peak, are an indicator of active oxygen vacancies on the surface [50,51].XPS results indicated that active oxygen vacancies were higher for the 1%Re4%Ni/Ce0.95Pr0.05O1.975(OA/OL = 0.28) compared to 5%Ni/Ce0.95Pr0.05O1.975catalysts (OA/OL = 0.24).Therefore, the bimetallic ReNi catalyst tended to display greater activity due to it producing more vacancies or defects.5%Ni/Ce0.95Pr0.05O1.975and 1%Re4%Ni/Ce0.95Pr0.05O1.975illustrated Ni 2p spectra mainly contributed by Ni 2+ species at around 855 and 856 eV with a minor content of Ni 0 species at around 853 eV (Figure 6b).All samples were reduced with 5%H2/N2 at 400 °C for 1 hour before XPS measurement.Ni species in the reduced catalysts were in the form of metallic Ni.Furthermore, the different Ni species co-existed due to the interaction with CeO2-based materials.Metallic Ni 0 was indicated to be the dominant active species in accelerating the reactants with content of 36.6% and 23.3% in 1%Re4%Ni/Ce0.95Pr0.05O1.975and 5%Ni/Ce0.95Pr0.05O1.975catalysts, respectively.Therefore, the increase in metallic Ni amount in the bimetallic ReNi catalyst implies a superior catalytic performance of Ni catalyst by the addition of Re.SEM analysis (shown in Figure 7) confirms there was almost no carbon deposition on the surface for the used 1%Re4%Ni/Ce0.95Pr0.05O1.975catalyst.Wang et al. [52] reported that more carbon was deposited and accumulated on the surface of monometallic Ni catalysts during steam reforming of biomass tar, whereas bimetallic NiFe catalysts suppressed the carbon deposition on the surface of the reacted catalyst.Therefore, using bimetallic catalysts could prevent coke formation on the catalysts by providing oxidation of the accumulated carbon.The carbon deposition of the spent catalysts was evaluated by TG analysis (Figure 8).
The oxidation of the carbon deposition in the air leads to weight loss.Small weight loss at SEM analysis (shown in Figure 7) confirms there was almost no carbon deposition on the surface for the used 1%Re4%Ni/Ce 0.95 Pr 0.05 O 1.975 catalyst.Wang et al. [52] reported that more carbon was deposited and accumulated on the surface of monometallic Ni catalysts during steam reforming of biomass tar, whereas bimetallic NiFe catalysts suppressed the carbon deposition on the surface of the reacted catalyst.Therefore, using bimetallic catalysts could prevent coke formation on the catalysts by providing oxidation of the accumulated carbon.materials.Metallic Ni 0 was indicated to be the dominant active species in accelerating the reactants with content of 36.6% and 23.3% in 1%Re4%Ni/Ce0.95Pr0.05O1.975and 5%Ni/Ce0.95Pr0.05O1.975catalysts, respectively.Therefore, the increase in metallic Ni amount in the bimetallic ReNi catalyst implies a superior catalytic performance of Ni catalyst by the addition of Re.SEM analysis (shown in Figure 7) confirms there was almost no carbon deposition on the surface for the used 1%Re4%Ni/Ce0.95Pr0.05O1.975catalyst.Wang et al. [52] reported that more carbon was deposited and accumulated on the surface of monometallic Ni catalysts during steam reforming of biomass tar, whereas bimetallic NiFe catalysts suppressed the carbon deposition on the surface of the reacted catalyst.Therefore, using bimetallic catalysts could prevent coke formation on the catalysts by providing oxidation of the accumulated carbon.The carbon deposition of the spent catalysts was evaluated by TG analysis (Figure 8).
The oxidation of the carbon deposition in the air leads to weight loss.Small weight loss at The carbon deposition of the spent catalysts was evaluated by TG analysis (Figure 8).The oxidation of the carbon deposition in the air leads to weight loss.Small weight loss at low temperatures (below 200 • C) was ascribed to the elimination of moisture and volatile species [53].The mass loss in the range of 200-400 • C was ascribed to the thermal decomposition of physisorbed carbonaceous species or soft-coke.A major weight reduction between 400 and 600 • C was due to the bulky carbonaceous products or hard coke on the used catalysts.The weight loss percentages of the bulky carbonaceous species on monometallic Ni and bimetallic NiRe catalysts were 11.6% and 6.2% for 5%Ni/CeO 2 and 1%Re4%Ni/Ce 0.95 Pr 0.05 O 1.975 , respectively, indicating that carbon decomposition decreases when use Pr as dopant and Re as metal additives.
low temperatures (below 200 °C) was ascribed to the elimination of moisture and volatile species [53].The mass loss in the range of 200-400 °C was ascribed to the thermal decomposition of physisorbed carbonaceous species or soft-coke.A major weight reduction between 400 and 600 °C was due to the bulky carbonaceous products or hard coke on the used catalysts.The weight loss percentages of the bulky carbonaceous species on monometallic Ni and bimetallic NiRe catalysts were 11.6% and 6.2% for 5%Ni/CeO2 and 1%Re4%Ni/Ce0.95Pr0.05O1.975,respectively, indicating that carbon decomposition decreases when use Pr as dopant and Re as metal additives.

Water-Gas Shift Activity and Stability
Figure 9 exhibits the %CO conversion of Ni/CeO2, Ni/Ce0.95Pr0.05O1.975,and ReNi/Ce0.95Pr0.05O1.975.From previous studies, it was found that further addition of Re does not further raise the rate of water-gas shift reaction, and the optimal content of Re that is enough to maximize the water-gas shift rate is 1 %wt.[54].When attention is drawn to the variation in Sm amount [12], it appears that the Ni catalyst with 5%Sm-doped CeO2 gives the highest water-gas shift activity.The enhancement of Sm content to 15 wt.% leads to a lowering of nickel dispersion.This result is due to the agglomeration of samarium at a high amount.Therefore, 5% doping amount and 1% of Re metal additives were used in this work to maximize the WGS performance.
For Ni/CeO2, the CO conversion started above 150 °C and ascended slowly to reach the maximum conversion of 84% at 350 °C.As observed, the highest CO conversion was achieved over a bimetallic NiRe/CePrO catalyst, reaching 96% CO conversion at 330 °C with a WGS activity higher than the activities of the monometallic catalysts.NiRe/Ce0.95Pr0.05O1.975has been determined as an excellent catalyst due to its high surface acidity, nickel metal dispersion, and nickel surface area which can enhance the concentration of CO adsorption on the catalyst surface.Furthermore, the NiRe/Ce0.95Pr0.05O1.975catalyst produced more oxygen vacancies, which could increase the redox ability, causing higher WGS activity.From previous studies, it was found that further addition of Re does not further raise the rate of water-gas shift reaction, and the optimal content of Re that is enough to maximize the water-gas shift rate is 1 %wt.[54].When attention is drawn to the variation in Sm amount [12], it appears that the Ni catalyst with 5%Sm-doped CeO 2 gives the highest water-gas shift activity.The enhancement of Sm content to 15 wt.% leads to a lowering of nickel dispersion.This result is due to the agglomeration of samarium at a high amount.Therefore, 5% doping amount and 1% of Re metal additives were used in this work to maximize the WGS performance.For Ni/CeO 2 , the CO conversion started above 150 • C and ascended slowly to reach the maximum conversion of 84% at 350 • C. As observed, the highest CO conversion was achieved over a bimetallic NiRe/CePrO catalyst, reaching 96% CO conversion at 330 • C with a WGS activity higher than the activities of the monometallic catalysts.NiRe/Ce 0.95 Pr 0.05 O 1.975 has been determined as an excellent catalyst due to its high surface acidity, nickel metal dispersion, and nickel surface area which can enhance the concentration of CO adsorption on the catalyst surface.Furthermore, the NiRe/Ce 0.95 Pr 0.05 O 1.975 catalyst produced more oxygen vacancies, which could increase the redox ability, causing higher WGS activity.Figure 10 presents the CO2 and CH4 selectivity of Ni/Ce0.95Pr0.05O1.975and ReNi/Ce0.95Pr0.05O1.975 in the temperature range of 300-500 °C.1%Re4%Ni/Ce0.95Pr0.05O1.975was exhibited to be an excellent catalyst in terms of WGS activity and selectivity of CO2 and CH4.Methane is an unwanted product because it is a precursor for coke formation and competes against H2 generation.As observed, 5%Ni/Ce0.95Pr0.05O1.975generates CH4 at low temperatures, whereas 1%Re4%Ni/Ce0.95Pr0.05O1.975 is highly selective toward the WGS reaction throughout the investigated temperature.Thus, the incorporation of Re onto Ni/Ce0.95Pr0.05O1.975increased CO conversion at the same time that it suppressed CH4 formation.The water-gas shift stability has been performed on the most active catalyst, ReNi/Ce0.95Pr0.05O1.975,under the feed mixture containing 5% CO, 10% H2O, and 85% N2 at  Figure 10 presents the CO2 and CH4 selectivity of Ni/Ce0.95Pr0.05O1.975and ReNi/Ce0.95Pr0.05O1.975 in the temperature range of 300-500 °C.1%Re4%Ni/Ce0.95Pr0.05O1.975was exhibited to be an excellent catalyst in terms of WGS activity and selectivity of CO2 and CH4.Methane is an unwanted product because it is a precursor for coke formation and competes against H2 generation.As observed, 5%Ni/Ce0.95Pr0.05O1.975generates CH4 at low temperatures, whereas 1%Re4%Ni/Ce0.95Pr0.05O1.975 is highly selective toward the WGS reaction throughout the investigated temperature.Thus, the incorporation of Re onto Ni/Ce0.95Pr0.05O1.975increased CO conversion at the same time that it suppressed CH4 formation.The water-gas shift stability has been performed on the most active catalyst, ReNi/Ce0.95Pr0.05O1.975,under the feed mixture containing 5% CO, 10% H2O, and 85% N2 at 300 °C.As shown in Figure 11, the ReNi/Ce0.95Pr0.05O1.975catalyst retained a high CO conversion of about 89% during the first 20 h on stream.Then, the CO conversion slightly decreases to 82% after 60 h of reaction.Hence, the bimetallic ReNi supported by Pr-doped CeO2 is resistant toward deactivation during a water-gas shift reaction.

Catalysts Preparation
A combustion technique was used to synthesize pure CeO2, and ceria-praseodymia mixed oxides supports with Ce(NO3)3.6H2O(Sigma-Aldrich, Pte. Ltd., Singapore) and Pr(NO3)3.6H2O(Sigma-Aldrich, Pte. Ltd., Singapore) as starting materials.Urea was utilized as a fuel to ignite the reaction.The redox reactions between NH2CONH2 and metal nitrates provide the exothermicity essential for the nucleation and growth of the metal oxide powders [55].Metal nitrate was mixed with urea using a stoichiometry between urea and metal nitrates as 1:2.5.Stirring a mixture obtained a homogeneous solution and then heating with a Bunsen burner until autoignition occurred.CeO2 and ceria-praseodymia mixed oxides powders were obtained by the thermal decomposition of nitrate and other organic compounds [56].

Catalyst Characterization
The specific surface areas of all catalysts were measured by N2 adsorption-desorption isotherms at 77 K using the BELSORP-MAX instrument (ITS Co. Ltd., Bangkok, Thailand).The samples were outgassed at 300 °C for 3 hours before the analysis.The Brunauer-Emmett -Teller method was utilized to estimate the specific surface areas of the catalysts.
X-ray powder diffraction (XRD) was performed using a PANalytical X'Pert Pro diffractometer (Malvern Panalytical Ltd., Malvern, UK) with the filtered radiation of a copper anode in the range temperature of 20-80°.The X-ray diffractograms were collected using the current of 40 mA and 40 kV with 0.02° per step and 0.

Catalysts Preparation
A combustion technique was used to synthesize pure CeO 2, and ceria-praseodymia mixed oxides supports with Ce(NO 3 ) 3 .6H 2 O (Sigma-Aldrich, Pte. Ltd., Singapore) and Pr(NO 3 ) 3 .6H 2 O (Sigma-Aldrich, Pte. Ltd., Singapore) as starting materials.Urea was utilized as a fuel to ignite the reaction.The redox reactions between NH 2 CONH 2 and metal nitrates provide the exothermicity essential for the nucleation and growth of the metal oxide powders [55].Metal nitrate was mixed with urea using a stoichiometry between urea and metal nitrates as 1:2.5.Stirring a mixture obtained a homogeneous solution and then heating with a Bunsen burner until autoignition occurred.CeO 2 and ceria-praseodymia mixed oxides powders were obtained by the thermal decomposition of nitrate and other organic compounds [56].
Ni(NO 3 ) 2 .6H 2 O (Alfa Aesar, Thermo Fisher Scientific Inc, Seoul, Republic of Korea) and NH 4 ReO 4 (Sigma-Aldrich, Pte. Ltd., Singapore) were used as the metal precursors for the preparation of Ni/CeO 2 , Ni/Ce 0.95 Pr 0.05 O 1.975 , and ReNi/Ce 0.95 Pr 0.05 O 1.975 via impregnation method.A solution of nickel and rhenium was added to ceria and ceriapraseodymia mixed oxides powders.All catalysts were dried at 100 • C for 12 h and calcined at 650 • C for 8 h.

Catalyst Characterization
The specific surface areas of all catalysts were measured by N 2 adsorption-desorption isotherms at 77 K using the BELSORP-MAX instrument (ITS Co. Ltd., Bangkok, Thailand).The samples were outgassed at 300 • C for 3 hours before the analysis.The Brunauer-Emmett -Teller method was utilized to estimate the specific surface areas of the catalysts.
X-ray powder diffraction (XRD) was performed using a PANalytical X'Pert Pro diffractometer (Malvern Panalytical Ltd., Malvern, UK) with the filtered radiation of a copper anode in the range temperature of 20-80 • .The X-ray diffractograms were collected using the current of 40 mA and 40 kV with 0.02 • per step and 0.5 s per step.The crystallite sizes of CeO 2 were estimated from the full width at half maximum of the strongest (111) reflection using the Debye-Sherrer equation.
Raman spectra were collected on Perkin-Elmer System 2000 FTIR/FT-Raman (Perkin Elmer, Rodgau, Germany) with argon ion laser irradiation at 532 nm wavelength and 10 mW maximum power.The spectra were recorded over the range of 100-1000 cm −1 using an operating spectra resolution of 1.0 cm −1 of Raman shift.
The H 2 chemisorption, Temperature Programmed Desorption of Ammonia (NH 3 -TPD), and H 2 -Temperature Programmed Reduction (H 2 -TPR) were performed using a catalyst analyzer BELCAT-B instrument (ITS Co. Ltd., Bangkok, Thailand) equipped with a thermal conductivity detector.The reduction behavior of the samples was studied by H 2 -TPR.The catalyst was first heated from room temperature to 120 • C in the He flow, maintained at 120 • C for 30 min, and cooled down to 50 • C under the He flow.The TPR measurement was performed from 50 • C up to 1000 • C with the rate of 10 • C/min under 5%H 2 in argon flow.NH 3 -TPD analysis was performed to investigate catalyst acidity.The catalyst was first heated from room temperature to 500 • C in argon flow and cooled down to 50 • C under argon.The catalyst was then exposed to pulse titration by using a loop of a known volume of NH 3 in Ar flow until saturation.NH 3 -TPD was finally carried out from 50 to 800 • C with a heating rate of 10 • C/min under argon flow.The H 2 chemisorption was performed to determine the surface area, particle size, and dispersion of Ni metal.The sample was evacuated in the He flow at 40 • C and then reduced at 400 • C for 1 h under H 2 flow (30 mL/min).The reduced catalyst was cooled down to 40 • C under helium flow and followed by volumetric H 2 chemisorption with pure hydrogen.The Ni surface area (S Ni ), Ni dispersion (D), and Ni particle size (d Ni ) were obtained from the instrument software based on the calculation by the following equation [57,58].
where V H is the chemisorbed H 2 volume (mL/g), Vm is the molar volume of H 2 (mL/mol), W is % wt. of nickel, M Ni is the atomic weight of nickel (g/mol), A Ni is the cross-sectional area of nickel atom (m 2 /atom), ρ is the density of Ni (g/mL), N A is Avogadro's number, and F is the stoichiometry factor (the number of active metal atoms to which one adsorbate gas molecule can attach).Scanning electron microscopy (SEM) was performed on an FE-SEM (HITACHI SU-8030, Hitachi High-Technologies Corporation, Tokyo, Japan) with high vacuum mode using secondary electrons and an acceleration of 30 kV.Energy dispersive X-ray spectroscopy (EDX) was used in conjunction with scanning electron microscopy for the elemental analysis.
Thermo-gravimetric analysis was performed using a Perkin-Elmer TGA/DTA 6300 instrument (Perkin Elmer, Rodgau, Germany) under an airflow rate of 100 mL/min.The content of carbon deposition on the used catalysts was investigated.The mass change in Ni-based catalysts was measured as a function of temperatures up to 800 • C with a heating rate of 20 • C/min.

Water-Gas Shift Activity
The water-gas shift activity was measured at the temperature from 100 to 500 • C. The catalyst (150 mg) was placed inside a fixed bed flow reactor (310 stainless steel, 0.6 cm outside diameter) between two layers of quartz wool.The catalyst was reduced under 5% H 2 in N 2 flow at 300 • C for an hour before the WGS activity testing.H 2 O was fed through a pre-heater using a syringe pump, whereas the flow rates of CO and N 2 were controlled by a mass flow controller.A mixed gas containing 5% CO, 10% H 2 O, and 85% N 2 was fed into the reactor.The total flow rate was maintained at 100 mL/min in all testing conditions.The composition of the gas mixture leaving the reactor was determined using an online Shimadzu GC-14B gas chromatography equipped with a thermal conductivity detector (TCD) and a ShinCarbon ST column.Argon is used as the eluent for a ShinCarbon ST column to detect the H 2 , CO, and CH 4 at the rate of 50 mL/min.The concentration of CO, CO 2 , and CH 4 at the outlet was repeated at least four times for each analysis.The water-gas shift activities can be calculated according to the following equation: %CO conversion = CO in − CO out CO in × 100 (5) where CO in is the inlet molar flow rate of CO (mol s −1 ) and CO out is the outlet molar flow rate of CO (mol s −1 ).

Conclusions
The influence of Re and Pr on the catalytic activity of Ni/CeO 2 was studied.The incorporation of Re and Pr into Ni/CeO 2 increased the WGS efficiency when compared with Ni/CeO 2 .An addition of Pr to Ni/CeO 2 reduced the crystallite size of CeO 2 , increased the BET surface area, and promoted higher dispersion of nickel on the CeO 2 surface.Furthermore, the role of rhenium on the water-gas shift performance of supported Ni catalyst was also considered.The results revealed that the addition of rhenium onto Ni/Ce 0.95 Pr 0.05 O 1.975 increased the catalytic performance toward the water-gas shift reaction and suppressed CH 4 formation.The role of rhenium in improving the catalytic activity was due to an increase in surface acidity, Ni surface area, and Ni dispersion, which facilitate CO adsorption on the catalyst surface.Additionally, the acidic character of the catalyst can accelerate CO 2 desorption, leaving behind free active sites for the adsorption of CO and H 2 O reactants in subsequent reaction cycles.Moreover, the enhancement of oxygen vacancy concentrations alerts the redox processes at the catalyst surface, which contributes to improving the WGS rate.

2 . 1 .
Catalysts Characterization X-ray diffraction patterns of CeO 2 , 5%Ni/CeO 2 , 5%Ni/Ce 0.95 Pr 0.05 O 1.975 , and 1%Re4% Ni/Ce 0.95 Pr 0.05 O 1.975 were illustrated in Figure 1.It suggests that the diffraction peaks of all catalysts correspond well to CeO 2 phases with a cubic structure (Joint Committee on Powder Diffraction File No. 43-1002).In addition, the weak peaks at 2θ about 37.1 • , 43.2 • and 63.2 • attributed to the NiO phases of Ni-based catalysts, suggesting that there was a small proportion of nickel oxide.The CeO 2 crystallite size of supported Ni catalysts was determined using the Debye-Scherrer equation ( C) after impregnation of Ni onto ceria support leads to the aggregation of ceria crystallites; thereby, the surface area decreases with a growth in CeO 2 crystallite size.However, Pr addition into 5%Ni/CeO 2 results in a decrease in the ceria crystallite size together with an increase in a specific surface area.The diffraction peaks of Ni/Ce 0.95 Pr 0.05 O 1.975 and ReNi/Ce 0.95 Pr 0.05 O 1.975 appeared at lower diffraction angles compared with the diffraction peaks of Ni/CeO 2 , indicating that Pr incorporation in the CeO 2 lattice enlarged unit cell.The enhancement of the unit cell for Ni/Ce 0.95 Pr 0.05 O 1.975

Figure 1 .
Figure 1.XRD patterns of Ni catalysts and CeO2 support.

Figure 1 .
Figure 1.XRD patterns of Ni catalysts and CeO 2 support.

H 2
chemisorption analysis was used to determine the Ni dispersion of supported Ni catalysts.It was found that an addition of Pr to Ni/CeO 2 increases Ni dispersion on the catalyst surface.Moreover, rhenium impregnation onto Ni/Ce 0.95 Pr 0.05 O 1.975 tremendously

16 Figure 3 .
Figure 3. Raman spectra of CeO2 support and Ni catalysts in the wide range of 100−1000 cm −1 (a) and the narrow range of 400−800 cm −1 (b).

Figure 3 .
Figure 3. Raman spectra of CeO 2 support and Ni catalysts in the wide range of 100-1000 cm −1 (a) and the narrow range of 400-800 cm −1 (b).

Figure 3 .
Figure 3. Raman spectra of CeO2 support and Ni catalysts in the wide range of 100−1000 cm −1 (a) and the narrow range of 400−800 cm −1 (b).

Figure 5 .
Figure 5. NH 3 -TPD of monometallic Ni and bimetallic NiRe supported by Pr-doped ceria.XPS characterization was used to investigate the surface chemical states of the catalysts.Figure 6a shows the O 1s XPS spectra of 5%Ni/Ce 0.95 Pr 0.05 O 1.975 and 1%Re4%Ni/Ce 0.95 Pr 0.05 O 1.975 catalysts.Three different types of oxygen species were detected in all samples.The detected peaks near 529 eV (O L ), 532 eV (O A ), and 533 eV (O H ) are attributed to lattice oxygen in metal oxide, chemically adsorbed oxygen on the surface, and a surface hydroxyl oxygen species, respectively.The ratios of O A /O L , which are calculated from the area of each peak, are an indicator of active oxygen vacancies on the surface [50,51].XPS results indicated that active oxygen vacancies were higher for the 1%Re4%Ni/Ce 0.95 Pr 0.05 O 1.975 (O A /O L = 0.28) compared to 5%Ni/Ce 0.95 Pr 0.05 O 1.975 catalysts (O A /O L = 0.24).Therefore, the bimetallic ReNi catalyst tended to display greater activity due to it producing more vacancies or defects.5%Ni/Ce 0.95 Pr 0.05 O 1.975 and 1%Re4%Ni/Ce 0.95 Pr 0.05 O 1.975 illustrated Ni 2p spectra mainly contributed by Ni 2+ species at around 855 and 856 eV with a minor content of Ni 0 species at around 853 eV (Figure 6b).All samples were reduced with 5%H 2 /N 2 at 400 • C for 1 hour before XPS measurement.Ni species in the reduced catalysts were in the form of metallic Ni.Furthermore, the different Ni species co-existed due to the interaction with CeO2 -based materials.Metallic Ni 0 was indicated to be the dominant active species in accelerating the reactants with content of 36.6% and 23.3% in 1%Re4%Ni/Ce 0.95 Pr 0.05 O 1.975 and 5%Ni/Ce 0.95 Pr 0.05 O 1.975 catalysts, respectively.Therefore, the increase in metallic Ni amount in the bimetallic ReNi catalyst implies a superior catalytic performance of Ni catalyst by the addition of Re.

Figure 8 .
Figure 8. Thermo-gravimetric analysis of the used catalysts.

Figure 8 .
Figure 8. Thermo-gravimetric analysis of the used catalysts.

Figure 9
Figure 9 exhibits the %CO conversion of Ni/CeO 2 , Ni/Ce 0.95 Pr 0.05 O 1.975 , and ReNi/ Ce 0.95 Pr 0.05 O 1.975 .From previous studies, it was found that further addition of Re does not further raise the rate of water-gas shift reaction, and the optimal content of Re that is enough to maximize the water-gas shift rate is 1 %wt.[54].When attention is drawn to the variation in Sm amount[12], it appears that the Ni catalyst with 5%Sm-doped CeO 2 gives the highest water-gas shift activity.The enhancement of Sm content to 15 wt.% leads to a lowering of nickel dispersion.This result is due to the agglomeration of samarium at a high amount.Therefore, 5% doping amount and 1% of Re metal additives were used in this work to maximize the WGS performance.For Ni/CeO 2 , the CO conversion started above 150 • C and ascended slowly to reach the maximum conversion of 84% at 350 • C. As observed, the highest CO conversion was achieved over a bimetallic NiRe/CePrO catalyst, reaching 96% CO conversion at 330 • C with a WGS activity higher than the activities of the monometallic catalysts.NiRe/Ce 0.95 Pr 0.05 O 1.975 has been determined as an excellent catalyst due to its high surface acidity, nickel metal dispersion, and nickel surface area which can enhance the concentration of CO adsorption on the catalyst surface.Furthermore, the NiRe/Ce 0.95 Pr 0.05 O 1.975 catalyst produced more oxygen vacancies, which could increase the redox ability, causing higher WGS activity.Figure 10 presents the CO 2 and CH 4 selectivity of Ni/Ce 0.95 Pr 0.05 O 1.975 and ReNi/Ce 0.95 Pr 0.05 O 1.975 in the temperature range of 300-500 • C. 1%Re4%Ni/Ce 0.95 Pr 0.05 O 1.975 was exhibited to be an excellent catalyst in terms of WGS activity and selectivity of CO 2 and CH 4 .Methane is an unwanted product because it is a precursor for coke formation and competes against H 2 generation.As observed, 5%Ni/Ce 0.95 Pr 0.05 O 1.975 generates CH 4 at low temperatures, whereas 1%Re4%Ni/Ce 0.95 Pr 0.05 O 1.975 is highly selective toward the WGS reaction throughout the investigated temperature.Thus, the incorporation of Re onto Ni/Ce 0.95 Pr 0.05 O 1.975 increased CO conversion at the same time that it suppressed CH 4 formation.

Figure 10
Figure 9 exhibits the %CO conversion of Ni/CeO 2 , Ni/Ce 0.95 Pr 0.05 O 1.975 , and ReNi/ Ce 0.95 Pr 0.05 O 1.975 .From previous studies, it was found that further addition of Re does not further raise the rate of water-gas shift reaction, and the optimal content of Re that is enough to maximize the water-gas shift rate is 1 %wt.[54].When attention is drawn to the variation in Sm amount[12], it appears that the Ni catalyst with 5%Sm-doped CeO 2 gives the highest water-gas shift activity.The enhancement of Sm content to 15 wt.% leads to a lowering of nickel dispersion.This result is due to the agglomeration of samarium at a high amount.Therefore, 5% doping amount and 1% of Re metal additives were used in this work to maximize the WGS performance.For Ni/CeO 2 , the CO conversion started above 150 • C and ascended slowly to reach the maximum conversion of 84% at 350 • C. As observed, the highest CO conversion was achieved over a bimetallic NiRe/CePrO catalyst, reaching 96% CO conversion at 330 • C with a WGS activity higher than the activities of the monometallic catalysts.NiRe/Ce 0.95 Pr 0.05 O 1.975 has been determined as an excellent catalyst due to its high surface acidity, nickel metal dispersion, and nickel surface area which can enhance the concentration of CO adsorption on the catalyst surface.Furthermore, the NiRe/Ce 0.95 Pr 0.05 O 1.975 catalyst produced more oxygen vacancies, which could increase the redox ability, causing higher WGS activity.Figure 10 presents the CO 2 and CH 4 selectivity of Ni/Ce 0.95 Pr 0.05 O 1.975 and ReNi/Ce 0.95 Pr 0.05 O 1.975 in the temperature range of 300-500 • C. 1%Re4%Ni/Ce 0.95 Pr 0.05 O 1.975 was exhibited to be an excellent catalyst in terms of WGS activity and selectivity of CO 2 and CH 4 .Methane is an unwanted product because it is a precursor for coke formation and competes against H 2 generation.As observed, 5%Ni/Ce 0.95 Pr 0.05 O 1.975 generates CH 4 at low temperatures, whereas 1%Re4%Ni/Ce 0.95 Pr 0.05 O 1.975 is highly selective toward the WGS reaction throughout the investigated temperature.Thus, the incorporation of Re onto Ni/Ce 0.95 Pr 0.05 O 1.975 increased CO conversion at the same time that it suppressed CH 4 formation.

Figure 10 .
Figure 10.Selectivity to CO2 and CH4 as a function of temperature over supported Ni catalysts.

Figure 10 .
Figure 10.Selectivity to CO2 and CH4 as a function of temperature over supported Ni catalysts.

Figure 10 .
Figure 10.Selectivity to CO 2 and CH 4 as a function of temperature over supported Ni catalysts.The water-gas shift stability has been performed on the most active catalyst, ReNi/Ce 0.95 Pr 0.05 O 1.975 , under the feed mixture containing 5% CO, 10% H 2 O, and 85% N 2 at 300 • C. As shown in Figure11, the ReNi/Ce 0.95 Pr 0.05 O 1.975 catalyst retained a high CO conversion of about 89% during the first 20 h on stream.Then, the CO conversion slightly decreases to 82% after 60 h of reaction.Hence, the bimetallic ReNi supported by Pr-doped CeO 2 is resistant toward deactivation during a water-gas shift reaction.

Table
).The calcination of Ni/CeO 2 catalyst at a high temperature (650

Table 1 .
BET surface area, crystallite size of CeO 2 , Ni dispersion, and Ni surface area of Ni-based catalysts.
a Calculated from the 111 diffraction peak broadening.b Estimated from N 2 adsorption at −196 • C. c Estimated from H 2 -chemisorption.

Table 1 .
BET surface area, crystallite size of CeO2, Ni dispersion, and Ni surface area of Ni-based catalysts.
a Calculated from the 111 diffraction peak broadening.b Estimated from N2 adsorption at −196 °C.c Estimated from H2-chemisorption.